Speed Vs. Energy: Which Is More Important In Crossbows?
August 14, 2017
The balance between speed and energy is crucial in crossbows. You need to know how to rig your bow to send arrows as fast as possible yet penetrate with power.
By Bob Humphrey
Our society is consumed with speed. We use high-speed internet, drive automobiles capable of traveling at twice the legal speed limit, and build jet airplanes that can fly faster than the speed of sound. Not surprisingly, when asked to choose between speed and energy in their crossbows, shooters opt for speed.
We demand the fastest bow-arrow combination possible, and that pushes crossbow makers to produce an ever faster- and flatter-shooting assortment of horizontal bows. They accomplish that through a combination of lighter broadheads and arrows, more powerful draw weights and longer powerstrokes.
Clear evidence is found in model names that often include a number indicating rated speed. You don't see numbers indicating foot-pounds of energy, or KE, in the model names, and a fair number of manufacturers don't even list the KE of their bows.
But nothing comes without a price. Satisfying the need for speed calls for a compromise, a sacrifice. Along the way you have to give up something and in the case of faster bows the cost is, yes, kinetic energy. However, there is a sizeable faction of bowhunters who lean toward the energy side. Speed vs. energy is one of those perpetual debates in hunting camps. Who is right? Which is more important? And is there something else we might not be considering?
Getting closer to the answers first requires some background in physics. According to the basic laws of physics, energy is neither lost nor gained; it is only transferred. By drawing your bow, you convert mechanical energy (ME) and store it as potential energy (PE) in the bow limbs. When you pull the trigger most of that PE is transferred to the arrow and converted to kinetic energy (KE), which propels it downrange.
You can calculate your arrow's KE and express it in foot-pounds. KE=0.5xMV squared. M is mass of the arrow and broadhead and V is velocity of the arrow.
The mechanics of a crossbow are fixed. You can't adjust draw weight and powerstroke like you can on a compound bow. Or as Gary Cornum, marketing manager for Easton Archery, puts it, "There's only so much energy you can put into the system. It's not like firearms where you can just add more powder to increase velocity."
In order to increase speed, V, you must reduce mass, M, by going to a lighter arrow. If you remember your 10th-grade science classes, reducing M also reduces KE. If, instead, you want to increase KE, you must increase M, arrow weight, which then reduces V. See the dilemma?
THE NEED FOR SPEED
The principal advantage of greater speed is flatter trajectory. With a flat trajectory, you will have a better chance of predicting arrow drop at distance and a better chance of making a good shot. As an example, if you shoot a 400-grain arrow from two bows sighted for 20 yards, one shooting 300 fps and the other at 350 fps, you will see about a 2-inch difference in point of impact at 30 yards. But at 50 yards there's a difference in drop of more than 8 inches. And that difference becomes even more pronounced with slower speeds and heavier arrows.
For a more extreme and comprehensive example, we can look at additional data compiled over roughly a six-year period by TenPoint's research and development engineering team based on repeatable, observable procedures rather than computations or computer simulations. Each test involved using the same bow zeroed at 20 yards to compare between different arrow weights. Looking at the extremes, a 370-grain arrow will drop about 20 inches at 50 yards compared to 30 inches for a 545-gain arrow.
Jake Miller from TenPoint's marketing department said the most staggering part was to see just how much that arrow can drop at different distances.
CROSSBOWS: HOW MUCH KINETIC ENERGY IS NEEDED?
Depending on what you are hunting, you'll want an arrow-and-broadhead setup that will push at least 25 pounds of energy into your game.
Small game, rabbits, groundhogs
Medium game, deer, antelope
Large game, elk, black bear, wild boar
- Toughest game, cape buffalo, grizzly bear
Source: Easton Archery
Clearly, a faster, flatter-shooting bow gives you an edge.
Mark Fedorak, engineering manager with Plano-Synergy, which makes Barnett crossbows, said if you're a target or competitive shooter, you'll want a lighter arrow that will get to the target as quickly as possible with the shortest amount of time in the air.
From a hunting perspective, that faster arrow helps compensate for numerous variables. At shorter ranges the difference may be negligible, or at least manageable, but for every variable you add — pounds, no rest, crosswind, nerves, string-jump — you increase the odds of a miss or a poor hit, which gets exaggerated the farther downrange the target is. As difference in drop and distance to target (as well as the other variables) increases, you eventually reach a point where visual range estimation is no longer an ethical option. You've got to use a laser rangefinder or risk missing or crippling your quarry.
So why even consider giving that up for more energy?
There are several reasons to give serious consideration to energy. As Fedorak points out, "There is no speed without energy. Speed is a byproduct of how much energy you're starting with."
As noted above, the ME you converted to PE remains static in the bow until the trigger is pulled. Most is then transferred to the arrow as KE (which influences how fast the arrow flies). The rest is dispersed as heat and friction or imparted into the bow and its limbs, wheels, riser, string and cables and other components as noise and vibration.
Energy transfer is more efficient with a heavier arrow so it will absorb more PE, meaning less KE will be transferred to the bow. Again, it's only a few foot-pounds but over time that takes a toll on your bow. Almost from the moment we pull the trigger we're losing speed and energy, so to a certain extent, the more we begin with the better.
Another reason is purpose. The target archer merely wants to drive his arrow into a relatively soft target at a relatively short distance. Hunters aim to kill. In addition to being a Level 2 U.S. archery coach, Mark Beck is a hunter and an engineer with Plano Synergy who has experience in designing both arrows and crossbows. He makes no secret which side of the argument he's on.
"I want more kinetic energy to drive that broadhead through," he said. "The arrow's job is to get the broadhead to the target and push it through. You want two holes."
He accomplishes that by using thicker, heavier and longer arrows with heavier heads.
"This goes back to the basics of archery," he said. "You get better spine reaction (with 22-inch arrows), and the extra 2 inches will not hurt anything." Meanwhile, the heavier heads result in a tip-heavy arrow that pulls better in the wind and improves front-of-center (FOC), optimizing the balance between energy and trajectory.
Gary Cornum seconded that notion, adding that if it came down to choosing between a heavier shaft or adding more weight to the front end with a heavier head or a brass insert, he would add the weight to the front to get the FOC advantages.
Beck seeks still further optimization by shooting a faster bow. Using Barnett's Ghost 410 as an example, he points out that going from a 400-grain arrow to 425 grains, you only lose 10 feet per second but gain a lot more energy, about 10 foot-pounds. "And there's virtually no difference in trajectory inside 30 yards," he said
MEET BIG MO
Perhaps the picture is becoming clearer. So it's time to complicate things a little more with a third, often overlooked variable. Momentum is also a product of mass and speed, and has a good deal to do with an arrow-broadhead combination's effectiveness.
You can calculate momentum (p) by the equation p = mass x velocity. What you need to know is: The heavier an object is, the greater the force required to slow it down. Because the force opposing or slowing down the arrow (air resistance) is relatively constant, a heavier arrow (with higher KE) will take longer to slow down. Though it's traveling slower than a lighter arrow, it will retain more speed and KE farther downrange. Ditto with the arrow's penetration into the game animal's flesh.
If you're a speed freak, you're probably already thinking, "Okay, that's great but how much energy do you really need?" That's a good question. KE is often used in describing ammunition, and with good reason. Bullets kill through both blood loss and trauma, the latter being a combination of extensive tissue damage and hydraulic shock caused by the sudden and traumatic transfer of KE upon impact — typically thousands of foot-pounds. For example, a 165-grain .308 Win. bullet will have somewhere in the neighborhood of 3,000 foot-pounds of KE at the muzzle, around 2,200 at 200 yards and 1,600 at 400 yards.
Arrows carry only a fraction of that much energy to their target. That's not an issue because, unlike bullets, broadheads kill by causing extensive hemorrhaging, that is, blood loss, from slice wounds. Therefore, an arrow needs only enough KE to penetrate the hide, flesh and ribs and enter vital areas. As Beck points out, a complete pass-through is preferable as it makes for a better blood trail and faster recovery, but it adds no more lethality to the wound. Any energy left in the arrow after the broadhead penetrates vitals is just surplus energy.
Now you know that a lighter arrow will get to the target faster and with flatter trajectory. It will minimize the effect of variables, like errors in range estimation or string-jumping game, and reduce the amount of time it is exposed to things like wind drift. But you also know that a lighter arrow will be more influenced by wind drift. It also will absorb less energy from the bow and carry less energy downrange into the target.
Knowing that, you can choose to maximize one variable, speed or energy, at the price of reducing the other. Or you can choose a bow, arrow and broadhead setup that maximizes each for your particular type of game, anticipated distances and conditions you are shooting in.